Head First 2D Geometry

Frank

Jill

Joe

finding missing angles

Something doesn’t add up!

Using the two different methods

for finding the mystery angles

gives us two different results.

And vertically opposite

angles are equal, so ? = 18º

Angles in a triangle add up to

180º so 81º + 86º + ? = 180º

? = 180º - 81º - 86º = 13º

This is not good. The whole investigation

could be compromised by bad math!

Joe: This is a disaster. How can there be two different answers?

It’s totally confusing.

Jill: Benny’s lawyer is gonna have a field day. I don’t believe it.

We’ve got the right guy in custody—it’s obvious he did it—and

we’re gonna have to let him go because of some messed up math!

Can we fix it?

Frank: The weirdest thing is that I’m looking again at the math

for either method and it looks good. I mean—corners on a

triangle always add up to 180 degrees, and vertically opposite

angles are always equal. Those are the rules.

Joe: I knew we were relying too much on these coincidences. My

guess is that these “rules” don’t always actually work!

Jill: I really don’t think that’s what it means. If we haven’t

messed up the calculations then the fact that we got two different

answers simply has to mean something else.…

What do you think it means?

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